Enhanced Tracking Performance Using Ultra-Tightly-Coupled GPS/INS Techniques

The need to provide for robust GPS navigation in challenging environments such as urban or dense vegetation is established. The use of ultra-tightly coupled techniques can significantly extend the tracking threshold of GPS receivers in these environments providing for robust navigation using code and carrier measurements at C/No levels previously unobtainable. The use of conventional tracking loops which employ fixed gains do not adapt well to time varying signal conditions in these environments. Under low C/No conditions typical of signal shadowing in the urbane or dense vegetation environments, carrier tracking loops break down at typically 18 dB Hz C/No. The fundamental reason for the loop break down is that at a low signal to noise ratio (SINR) into the loop there is virtually no restoring force to any loop perturbation. The only way to recover signal to noise ratio is to integrate longer and use narrow bandwidths which results in correlated measurement problems. Using techniques which operate on the I and Q data to directly produce residuals that are input to a Kalman filter eliminates the need for conventional tracking loops. These techniques provide optimal processing gain while retaining the Kalman Filter optimality requirements of uncorrelated measurement errors. These techniques have been successfully implemented into a miniaturized GPS receiver and demonstrated in handheld and weapon system applications providing enhanced tracking performance. When coupled with low cost inertial sensors, these techniques can provide reliable tracking at levels down to 2 dB Hz C/No. This paper provides an overview of the techniques used and the implementation of these techniques into a miniaturized GPS receiver developed for weapon system applications. This paper also addresses the results from a high fidelity GPS receiver simulation and actual laboratory and field testing of this GPS receiver. 1.0 INTRODUCTION Unaided GPS receivers used in the ground and urban environment for mobile and handheld applications typically use conventional code and carrier tracking loops. Delay lock loops (DLLs) are used for tracking code phase and phase lock loops (PLLs) are used to track carrier phase. These tracking loops must have the bandwidth and stability to reliably track the GPS signal through platform dynamics and clock instabilities and varying C/No conditions caused by signal shadowing and foliage density. Tightly coupling the GPS receiver to an Inertial Measurement Unit (IMU) using conventional code and carrier tracking loops have demonstrated good code and carrier phase tracking performance under high vehicle dynamic by aiding these loops with delta velocity and phase measurements from the IMU. Tightly-coupled GPS receivers use the IMU data to remove most of the vehicle dynamics so the receiver only has to track through the residual motion. Weiss (1995) This allows the receiver tracking loops to have lower bandwidth, resulting in improved tracking performance. However, the use of conventional Lewis, D.E. (2007) Enhanced Tracking Performance Using Ultra-Tightly-Coupled GPS/INS Techniques. In Military Capabilities Enabled by Advances in Navigation Sensors (pp. 28-1 – 28-14). Meeting Proceedings RTO-MP-SET-104, Paper 28. Neuilly-sur-Seine, France: RTO. Available from: http://www.rto.nato.int. Enhanced Tracking Performance Using Ultra-Tightly-Coupled GPS/INS Techniques 28 2 RTO-MP-SET-104 UNCLASSIFIED/UNLIMITED UNCLASSIFIED/UNLIMITED tracking loops which employ fixed gains do not adapt well to time varying signal conditions in these environments. Under low C/No conditions typical of signal shadowing in the urbane or dense vegetation environments or in the presence of interference, carrier tracking loops break down at typically 18 dB Hz C/No. Applications using an IMU when Ultra-Tightly Coupled (UTC) to the GPS receiver have demonstrated significantly improved tracking performance through further reduction of the receiver tracking loop bandwidths. Jaffe and Rechtin (1955) These Ultra-Tightly Coupled techniques when employed in applications without the use of an IMU have also shown improvements in the receiver’s tracking performance under low C/No conditions . 1.1 Loosely Coupled and Tightly-Coupled GPS Architectures Loosely coupled GPS/INS systems work reasonably well in fixed or static environments. These systems used conventional phase and frequency tracking loops and increasing the tracking performance of these systems could be obtained by narrowing the tracking bandwidths and lengthening the integration times to improve the receiver’s carrier to noise (C/No). However, in a more dynamic C/No environment cause by signal shadowing in an urbane environment or due to heavy foliage other techniques may be more effective. It may also become necessary to “aid” the tracking loops due to platform dynamics to maintain performance giving rise to “tightly coupled” GPS/INS systems. Under high C/No conditions, both looselycoupled and tightly-coupled GPS receivers perform well. Under low C/No conditions narrow tracking loop bandwidths need to be employed, resulting in temporally correlated noise which is a sub-optimal solution when viewed from the point of view of the Kalman filter. Narrow bandwidths also produce a correlation effect between measurements and process noise which tends to have a destabilizing effect on system performance. As noise increases the carrier tracking loop breaks down at approximately a C/No of 18 dB-Hz. The reason for the loop break down is the low signal to noise ratio (SINR) into the Costas loop (product of C/No and the coherent integration time [C/No) ⋅ Ti = SINR] which is the signal to noise ratio prior to the squaring operation). When this happens, the information loss through traditional non-linear loop error discriminates, such as the arc tangent function, becomes prohibitive, resulting in virtually no restoring force to loop perturbation. The only way to recover signal to noise ratio is to integrate longer and use narrower bandwidths. Conventional tracking loops employ fixed gains which do not “adapt” well to time varying signal conditions. Data wipe off techniques is one method being employed to improve signal to noise ratio. Since I and Q data input to the baseband algorithm is bi-phase shift keyed (BPSK) modulated at a 50 Hz rate, the signal polarity can change at a 50 Hz rate. Coherently adding I or Q signal samples over beyond the 50 Hz rate can result in signal cancellation. Data wipe off techniques use a priori estimates of the 50 Hz data stream to remove this effect, thus allowing coherent integration which results in improved tracking performance. GPS receiver tracking loops maintain carrier phase lock and code lock at C/No levels of approximately 18 and 13 dB respectively. The outputs of these tracking loops produce range and delta (∆) range measurements to a Kalman filter. Below these C/No levels, complete loss of GPS measurements occurs, resulting in an inertial only system drifting in a divergent fashion. Figure 1 depicts the architecture for a tightly coupled GPS/INS system. The functions to the left of the dotted line are performed typically in hardware and the functions to the right are performed in software. The GPS satellite signal is received by an antenna, is down converted to baseband and digitized by an analog to digital (A/D) converter prior to the functions shown in Figure 1. The digitized signal is then provided to a digital GPS receiver. The digital GPS receiver function provides the replica code generation and correlators. LOS geometry to the satellites is computed using the Earth Centered Earth Fixed (ECEF) Position and velocity of the receiver, GPS time, ephemerides and known satellite position and velocity information. The LOS Geometry outputs aiding information in the form of range and range rate to the replica code generator, the tracking loops, and the residual form functions to remove the effects of receiver Enhanced Tracking Performance Using Ultra-Tightly-Coupled GPS/INS Techniques RTO-MP-SET-104 28 3 UNCLASSIFIED/UNLIMITED UNCLASSIFIED/UNLIMITED motion. The replica code generator uses the aiding information to adjust the time position of the replica code such that it aligns with the satellite generated code. The hardware then generates the I and Q phase outputs that are fed to the tracking loops to adjust the replica based on the observed error. The tracking loop software then determines whether the replica is early, late or prompt based on the I and Q information. This is then used to adjust the time position of the replica. The range measurements are then formed by the tracking loop, and residuals are created and fed to the Kalman filter. The Navigation function uses the output of the Kalman Filter and the IMU to update the receiver’s current location and velocity. The performance of a Tightly-Coupled GPS/INS is susceptible to a number of error sources including receiver clock instabilities, IMU instabilities, IMU interface errors, and lever arm compensation. Receiver clock and IMU data are used directly by the receiver without any filtering or compensation, and therefore receiver track loop bandwidth must be wide enough to encompass these errors in order for the receiver to provide accurate and reliable position data. Replica Code Generation & Correlators Tracking Loops Kalman Filter Navigation IMU LOS GEOMETRY ∆θ & ∆V ECEF Position and Velocity Hardware Software I δx Form Residual DR R δR δDR Line of Sight Aiding Q Figure 1. Tightly-Coupled GPS/INS System 1.2 Ultra-Tightly Coupled Architecture Ultra-Tightly Coupled GPS/INS techniques provide improvements in the tracking performance of the GPS receiver under low C/No conditions. As shown in Figure 2, the Ultra-Tightly Coupled GPS/INS architecture eliminates the traditional tracking loop function and instead processes I and Q data directly into the Kalman filter. Enhanced Tracking Performance Using Ultra-Tightly-Coupled GPS/INS Techniques 28 4 RTO-MP-SET-104 UNCLASSIFIED/UNLIMITED UNCLASSIFIED/UNLIMITED 001-4129586 Signal Processing I & D KF NAV I&Q Resid δ x ∆Θ & ∆ V Guidance ECEF Position and Velocity LOS Aiding LOS Geometry IMU Figure 2. Ultra-Tightly Coupled GPS/INS Architecture In the Ultra-Tightly Coupled Architectures, the intermediate tracking loops are replaced by an integrate and dump (I & D) operation which then provides residuals directly into the Kalman filter. Greenspan (1996) Using this method, each measurement is independent from sample to sample and loop closure is accomplished by the Kalman filter (and navigation function) as opposed to the tracking loops which are also closed loop systems. This process takes advantage of certain mathematical/statistical properties of the residual estimates used to enhance SINR and thus provides a more stable system at lower C/No levels. The integrate and dump algorithm, as shown in Figure 3, inputs the I and Q signals from the GPS signal processor at a predetermined rate. These signals are added up coherently (in phase) over a designated time frame (Ti). This time frame is determined as a function of LOS range rate covariance calculated from Kalman filter covariance in order to determine the optimum pre-detection integration time Ti. The outputs from each coherent summation Σ( ), cross and dot products are formed at the Ti rate and the cross and dot products are then summed over a different time interval of length Tk. This time, Tk, is chosen to a targeted or desired signal to noise ratio and is determined by SINR estimates computed by the GPS receiver to provide a pair of second summations or integrals, one for the dot product signal and one for the cross product signal. Then an arc tangent function of the summed cross products and dot products is taken and divided by the integration time (Ti) to obtain the range rate residual directly. This residual is then input to a Kalman filter.